Larger DC to AC inverters are coming into general use. The magnitude of the DC current needed to supply these larger inverters is becoming problematic. In vessels there are many motor driven pieces of equipment that could be advantageously driven by motor speed controllers, but the use of dedicated motor speed controllers has been restricted by cost.
An inverter producing 50 or 60 Hz sine-wave mains power will draw current from its DC supply in the form of a sine squared waveform with the peaks repeating at a rate of 100 or 120 Hz. (Note: Mains power has a mains frequency and voltage that will be known to those skilled in the art. Mains frequency means the frequencies used to transmit AC current from an electrical generator through an AC power transmission network to electrical loads. The frequencies currently in use are 16.7 Hz, 50 Hz, 60 Hz and 400 Hz, but many other frequencies were used in the early days of electrical power generation. In most parts of the world, mains voltage is 115V (at 60 Hz) or 230V (at 50 Hz)). For an ideal inverter with no losses, the peak DC current will be twice the average current and the minimum current will be zero. This is equivalent to a DC current flow (equal to the average current) with a large AC current superimposed on it. The peak-to-peak amplitude of the AC component is equal to twice the average current. This AC components is often called ripple current. In practice an inverter will normally contain a reservoir capacitor across its DC supply terminals that will tend to supply some of the ripple current. In most commercially available inverters the reservoir capacitor is not large enough to materially reduce the mains frequency related AC component drawn from the DC supply. When the current draw by an inverter comes from an energy store, the ripple current, in conjunction with the equivalent series resistance (ESR) of the energy store, produces a ripple voltage at the energy store terminals. Ripple voltage can cause a number of problems in other equipment connected to the same energy store. Some audio equipment is susceptible to ripple voltage on its supply connection and will produce audible interference at the ripple frequency. If the ripple voltage is large enough, the voltage dips corresponding to the peaks in the current waveform may be low enough to trip the under-voltage detect function of some electronic equipment, causing the equipment to reset. A DC current with a superimposed ripple current has a higher root mean square (RMS) value than just the DC current by itself. Thus, the heat produced when the combined currents flow through a resistance is greater than when just the DC current flows, even though the average current in both cases is the same. This effect will occur in both the ESR of the energy store and in the resistance of the cables between the energy store and the inverter. At present the lowest cost energy store is one constructed from lead-acid rechargeable batteries. The ripple current from an inverter system influences the chemical reactions that take place inside the cells of a lead-acid battery. A lead-acid cell is constructed with the lead compounds formed into plates, which are surrounded by an acid electrolyte. The electrolyte takes part in the chemical reactions that either generate or store electric current. In order for the lead compounds in the interior of the plates to react, the electrolyte must diffuse through from the outside of the plate. This diffusion process is slow relative to the current changes brought about by the ripple current, which causes the chemical reactions to be concentrated on the outside of the plates whenever the cell charge or discharge current has a large ripple current component. There are various wear-out mechanisms that limit the number of times that the lead compounds can transition between their charged and discharged states, so the tendency for the reactions to occur preferentially in a portion of the lead compounds results in a reduced cell life. Almost all cell chemistries involve reactions between solid active materials and a liquid electrolyte and so are highly likely to show the same kind of reduction in life when subjected to high ripple currents.
It is common for a long term DC energy store suitable for connection to a multi output inverter according to the present invention to be charged from an alternator on the main engine of a vessel or RV. One of the problems associated with these alternators is the generation of a “load dump” transient over-voltage if the energy store is disconnected from the alternator while the alternator is supplying a significant current. A load dump is caused because the alternator charge control circuit is not capable of quickly reducing the current in the field coil that sets the alternator output, so the alternator output voltage rises up towards its maximum open circuit voltage which can be up to six to eight times the nominal system voltage. Many electronic products on the market are not designed to withstand this type of over-voltage event. However, they can be protected if there is a device connected to the DC distribution wiring that is capable of absorbing the excess energy from the alternator until the field coil current has been cut back. This is no simple task because of the large amount of energy involved.
On a boat it is common for equipment to be powered by brushed DC motors which are supplied with current from a battery bank. Examples include bow thrusters, stern thrusters, anchor winches, electric sheet winches, water makers, and refrigeration compressors. This type of motor has many problems, such as the need to regularly maintain and replace the brushes, the size and weight of the supply cables, the high weight of the motor, the high turn on current surge, the high start up acceleration of the motor, the sensitivity to voltage drops, and fixed speed operation.